Accurate in situ Writer output waveform monitoring (i.e., output waveform monitoring that occurs without interrupting the normal state of a system) is difficult to implement in disc drives for a number of reasons. Such reasons include impaired head disc assembly (HDA) access, the fragile nature of the preamplifier-to-head interconnects, and/or probe loading effects. Several authors have reported application of on-chip eye monitors to serial backplane and wireline communication to solve such problems.
Monitoring capability is valuable to allow monitoring of a write and/or optimization on all heads. Conventional approaches sometimes attempt to introduce test probes into the confined HDA. However, test probes introduce undesirable loading which change the HDA performance. Further, as writer risetimes decrease, laboratory evaluation and production final test of writers is increasingly problematic due to test fixture parasitics. Difficulty in replicating the exact disc drive environment where the writer is used is also a problem. In non-testing environments such as non-laboratory settings, writer performance monitoring sometimes improves yields by allowing direct inspection of such performance aspects as risetime and transmission line reflections.
Other conventional writers employ an adaptive parameter setting that can be mechanized using software-based LMS algorithms implemented in the microcode of the drive to force the measured waveform to a desired shape or by measuring bit error rates. Each head-interconnect-preamplifier channel in a disc drive can automatically be optimized during the final test phase, by variation of parameters available in the chosen writer design. In such conventional writers, optimizable parameters include overshoot amplitude, duration, and steady-state write current.
Conventional implementations for characterizing preamplifier writers use comprehensive measurements in a laboratory setting. Such an approach is not easily implemented in a mass-production environment or to analyses at a particular customer site. A further disadvantage of such an approach is that measurement probes inevitably introduce circuit loading and affect results. A customer application environment is difficult to replicate.
It would be desirable to implement a write performance monitor in an optical or magnetic storage device that provides monitoring without affecting results.